1. Field of the Invention
This invention relates to specific binding assays for analytes in various fluids and particularly to assays that use agglutination as a means of detecting binding.
2. Description of the Background
The use of immunoassays to detect the presence of analytes in various clinical samples has grown explosively in the last few years. Many of these assays have relied upon the use of a radioisotopic label to detect positive reactions between analytes and antibodies. Although the potential hazard for using these reagents is relatively low because of the small amount of radioactivity involved, the equipment required is expensive and complicated and precautions must be taken in disposal of the spent reagents. Reagent instability is also a problem.
The use of enzymes and other non-radioactive labels to produce color reactions has eliminated many of the problems associated with radioactivity. However, color-forming reactions are not appropriate in all media in which assays need to be carried out, such as whole blood. Furthermore, dilution of sample is inevitably required.
Agglutination assays do not require expensive detection equipment, since agglutination can be detected visually. This type of assay was initially developed to determine the presence of specific antigens on red blood cells, for example as with blood typing. In the most commonly used general technique based on agglutination, antibodies are attached to the surface of a solid particle, typically a latex particle, and mixed with a sample. If antibodies on two or more particles can react with an individual polyvalent antigen, a crosslinking will occur between the particles, resulting in the agglutination and/or precipitation of particles. Such agglutination can be detected visually or with instruments. However, such agglutination assays require the labeling of a different latex or other solid particle with a specific immunoglobulin for each type of analyte being detected. Particulate reagents are difficult to make and handle.
Additionally, visually read agglutination assays are non-objective and are usually qualitative. Although it is possible to carry out instrumental (objective) reading of agglutination, the instruments are generally complex and expensive. Furthermore, when whole blood is used as a sample, it is often necessary to remove red blood cells if conventional techniques are used, which increases handling and reduces the convenience of such assays.
Accordingly, there remains room in this field for improved techniques based on agglutination, particularly techniques which do not require the making of different particulate reagents for each analysis or require the removal of red blood cells from whole blood samples and which are readily adaptable to instrumental quantitation.